The ancient underwater volcanoes about 250km off the coast of Sydney as sonar mapped by the Investigator research vessel. Link to video

Four enormous underwater volcanoes, thought to be about 50m years old, have been discovered off the coast of Sydney by a team of scientists who were looking for lobster larvae.

The volcano cluster was spotted through sonar mapping of the sea floor by Investigator, Australia’s new ocean-going research vessel, about 250km off the coast.

The four volcanoes are calderas, large bowl-shaped craters caused when a volcano erupts and the land around it collapses. The largest is 1.5km across the rim and rises 700m from the sea floor. The 20km-long volcano cluster is nearly 5km underwater.

Professor Iain Suthers, a marine biologist at the University of NSW, said the volcano discovery was made when the team was searching for nursery grounds for larval lobsters.

“My jaw just dropped,” Suthers told Guardian Australia. “I immediately said, ‘What are they doing there and why didn’t we know about them before?’ It really backs up the statement that we know more about the surface of the moon than our sea floor.

“I’m elated. We went there to look at eddies in the east Australia current and it was completely serendipitous to find this volcano cluster. We can only just imagine what will be around the corner if we continue to scan this area.”

Scientists believe the volcanoes were created by a series of shifts in geological plates that caused Australia to split from New Zealand. Suthers said the area was thought to be “billiard-table flat” but the enhanced mapping capability of the Investigator unveiled the calderas.

The 94-metre Investigator was commissioned by the CSIRO in 2009 via $120m from the federal government. The vessel, which undertook its first sea tests in March, can map the seafloor at any depth, whereas its ageing predecessor, the Southern Surveyor, was limited to 3,000 metres.

Professor Richard Arculus, an igneous petrologist and volcano expert at the Australian National University, said the Investigator’s mapping ability has unveiled an “enormously exciting” discovery.

“They tell us part of the story of how New Zealand and Australia separated around 40m to 80m years ago, and they’ll now help scientists target future exploration of the sea floor to unlock the secrets of the Earth’s crust,” he said.

The team of 28 scientists, led by Suthers, included researchers from NSW, Latrobe, British Columbia, Sydney, Auckland, Technology Sydney and Southern Cross universities. The voyage left Brisbane on 3 June and arrived in Sydney on 18 June.

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Suthers said it was “inevitable” that other undiscovered volcanoes were in the region, but the Investigator has funding to operate at sea for only 180 days a year. For the rest of the year it is tied up at a wharf in Hobart.

“We should thank Canberra for the funding we do have but it’s frustrating to build a state-of-the-art vessel only to have it sitting in a wharf for six months of the year,” Suthers said.

“This is a vessel that Australia has been crying out for for decades. It’s an incredibly stable vessel for those of us who are seasick. Usually when you’re hit by four-metre waves you lose a couple of days of research because you’re vomiting.”

A spokeswoman for Ian Macfarlane, the industry and science minister, said the shortfall was because Labor “left absolutely no money in the budget” to operate the Investigator.